Scientific Method —

Cheap and cheerful gravity wave detector on the horizon

We've been looking for gravity waves predicted by the general theory of …

Gravity waves seem to be the ultimate in hard-to-detect phenomena. Currently, we have a couple of rather large laser interferometers and a giant suspended pendulum looking for them, so far without success. Furthermore, astronomers have been busy observing variations on pulsar frequencies and the like as a way to use astronomical objects as gravity wave detectors. Then there are the next-generation detectors, all in various stages of development, that include space-based observatories, among other approaches.

Now we have another detector to add to the list. A team of researchers from Japan and Germany have proposed a new way to detect gravity waves. The big news is that, compared to the others, it is a lot simpler, probably a lot cheaper to build, and nearly as sensitive.

First questions first: how do we know that gravity waves exist? The general theory of relativity, which is, at heart, a theory of gravity, predicts them. In general relativity, changes in mass at a location cause space and time to stretch and compress. Rather like sound waves, compressing space-time causes stretching in neighboring regions and vise versa. In this way, a moving distortion in space-time is created. We can detect these by measuring the response of a mass to the distortions in space-time.

Now, it's possible that gravity waves don't exist, but for that to be true, much of general relativity would have to be wrong. Or maybe I should say more wrong, or even very, very wrong. So much so that it would be an amazing coincidence that all other tests of general relativity support it.

Nevertheless, the current generation of detectors aren't likely to be sensitive enough to detect gravity waves. And, once we have ones that are sensitive enough to detect gravity waves from relatively frequent astronomical events, they will be insensitive to some of the most interesting: low frequency gravity waves left over from the Big Bang.

The oldest thing that we can see is the cosmic microwave radiation. And, even though this stuff is pretty old, it comes from the moment when the Universe cooled enough for the plasma of charged particles to condense into atoms. In other words, the oldest light in the universe comes from the moment just after all the interesting stuff occurred. On the other hand, gravity waves were not absorbed by that plasma, so the equivalent—a cosmic gravity wave background—would allow us to see further back in time.

We want to see this stuff, but it is difficult. New detectors will most likely detect gravity waves from recent events, but what we really want are arrays of detectors that are most sensitive at very low gravity wave frequencies (less than one oscillation per second). Enter TOBA, the torsion bar detector, the cheap and cheerful gravity wave detector.

TOBA is like any other gravity wave detector in that it tries to measure distortions in space by observing their effect on a test mass. In this case, the detector consists of a long bar that is gently twisting back about its center. If a gravity wave should impinge upon it, the bar experiences a twisting force, called a torque, that either slows or speeds up the motion. This can be detected by carefully measuring the positions of the ends of the bar as a function of time.

A pair of these, oriented at right angles to each other and twisting out of phase with each other, provide an excellent detection system, because things like earthquakes effect both in a similar manner, while gravity waves do not, which provides a natural filter against a lot of ground-based noise sources.

Another advantage is that the torsion bar is twisting at its own frequency, and gravity waves are measured as deviations from that frequency. This means that, instead of trying to measure deviations from a stand-still, we are measuring changes to a non-zero frequency. This is important, because the further you move away from the zero-frequency point, the less intrinsic noise there is in the measurement.

The researchers estimated the sensitivity of a 10m long aluminum bar that weighed just 7600kg, finding that it should have a sensitivity of around 10-14-10-191/Hz1/2. In the high frequency range, this is no better than what is expected for the upgraded versions of the LIGO detector, but the low frequency sensitivity is very good.

In contrast to LIGO, though, a 10m object is small and relatively cheap to build. It could be the key element to the gravity wave detector array. If it were made smaller, it could be sent into space as well. If construction is funded, you can guarantee that you will be hearing about it again.

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku

Really interesting article. Just a quick point on the writing, hope you don't mind.

" In this case, the detector consists of a long bar that is gently twisting back about its center."Should this read" In this case, the detector consists of a long bar that is gently twisting back and forth about its center."?

"Well, um, using layman's terms... Use a retaining magnetic field to focus a narrow beam of gravitons - these, in turn, fold space-time consistent with Weyl tensor dynamics until the space-time curvature becomes infinitely large, and you produce a singularity. Now, the singularity... "

I do think this research is incredibly important despite my Event Horizon quote. I thought the quote was better than "hmm" or "ahh" or "ohh".

I think it's always important to note for these cases where you haven't detected what you want to detect - it may seem valueless to the layman, but it's very useful for constraining upper/lower limits on theoretical values and therefore steering scientists in the direction of a correct theory among many (or at least away from the wrong one).

Why? I mean, if gravity comes as a side-effect so to speak of mass (my layman's understanding, hence the if), it shouldn't ever be able to travel faster than mass, which shouldn't ever be able to travel at the speed of light?

Why? I mean, if gravity comes as a side-effect so to speak of mass (my layman's understanding, hence the if), it shouldn't ever be able to travel faster than mass, which shouldn't ever be able to travel at the speed of light?

Gravitational waves themselves do not have (rest) mass, and do not need mass to propagate. This is why they travel at the speed of light. This is the same reason that light travels at the speed of light, even though the electric field is originally generated by charged particles which have mass and travel at sub-light speeds.

Hum... this particular setup is quite different from Weber bars, although it is still using resonance... but then, that is the best thing you can do to get high resolution measurements of "distance" perturbation (between quotes, because it would be more accurately named a perturbation of the local space-time curvature).And even though Weber's result were though inconclusive at the time, the controversy is not yet resolved... one way or the other.

Of course, an other way to look at it is that precision manufacturing and measurements have considerably improved since the '60, which means that if exactly the same apparatus was built today, its sensitivity would probably be much higher... and *that* is definitely a better plan.

First questions first: how do we know that gravity waves exist?...Now, it's possible that gravity waves don't exist

You don't mention the Hulse-Taylor binary. It's not a direct observation, only an indirect one, but it is a magnificent one. We're pretty sure that graviational waves exist, not just because GR predicts them, but also because we've seen collapse of the orbit of a pulsar in a binary system, in exact agreement with GR's prediction of energy loss due to gravitational radiation.

When I first saw that graph in a textbook, the caption said that the error bars weren't displayed on the plotted points because they are smaller than the thickness of the interpolated drawn curve. The text described it as one of the greatest triumphs of modern science.

Taylor and Hulse did their measurement in 1974 and were awarded the Nobel Prize in 1993.

Some guy named Weber already tried aluminum bars back in the late '60s. What makes this plan better?

Shot-noise limited position detection sensitivity and better thermal control for the most part.

Also, traditional weber bar style detectors are only highly sensitive near the ringing frequency of the bars, which is typically in the range of 900 Hz to 3 kHz. A given detector having maybe half an octave of useful sensitivity. It is going to be very hard to beat LIGO, much less advanced LIGO in the 100 Hz-1 kHz band. I can't read this article from home, but the interesting thing seems to be the use of a torsional pendulum configuration, along with a crossed pair of detectors to measure low frequency vibrations that are inaccessible to LIGO (LISA: a space based gravitational wave interferometer is planned to work better here, but space is expensive).

Probably a key part of this is also improved suspension over traditional bar detectors. In a bar detector, you want to put your supports at the nodes of the mechanical ringing motion. Ideally then, the supports don't move at all and their damping doesn't contribute to your noise floor. Of course, when you are talking about measuring 10^-20 m, that is a dramatic over-simplification, but the point remains, bar detectors are designed to limit the effect of the mechanical supports. With a pendulum, you can't do that: most or all of the deformation is in the supports, and any damping there will limit your sensitivity. Losses in the steel support wire of the LIGO test masses is one of their sensitivity limits, and for Advanced LIGO they have designed a new support based on silica fibers with a very carefully designed attachment to the mass that lowers their thermal noise dramatically.

I only take issue with the interference. Since both bars cannot be mounted with the same center of mass, (some 5m apart, minimum) terrestrial waves will interfere. However, these should differ by order of magnitude and additional measurements should be able to measure the interference and back it out.

Well, this may be a good start. A few decades from now we may have the ability to listen in on E.T.'s conversations. You KNOW that they gave up on radio eons ago, didn't you? They all use SHF gravity wave transceivers now. Fit in a pouch on your hip, or whatever those E.T.s use for a hip.

Something that's always bugged me about any experiment that involves "seeing back to the Big Bang": wouldn't anything radiated from the Big Bang have long since passed us by?

Not unless the whole universe has passed us by. It is weird to think about, but the Big Bang did not happen at a specific location "over there". This is why there is the background radiation we can currently see -- gravity waves being something else we would like to see.

Something that's always bugged me about any experiment that involves "seeing back to the Big Bang": wouldn't anything radiated from the Big Bang have long since passed us by?

Don't think of the universe like the surface of a balloon. Think of it like an expanding gas cloud: there's stuff further "out" than we are, and there's stuff further "in" then we are. In either direction, there's stuff that's so far away we're just now receiving what it was radiating when the universe became transparent.

I just find myself thinking that these sensors depend on changes happening over time, yet my layman understanding of relativity is that gravity can play tricks with time. So what is the chance that a gravity wave hides behind "a trick in time"?

Don't think of the universe like the surface of a balloon. Think of it like an expanding gas cloud: there's stuff further "out" than we are, and there's stuff further "in" then we are. In either direction, there's stuff that's so far away we're just now receiving what it was radiating when the universe became transparent.

That's going to take some further explanation. Given the highly compressed nature most of us are taught about for the whole concept of the big bang it seems strange that radiation from the event, traveling out at faster than the speed of light could be reaching massed particles (us) traveling out at slower than the speed of light now. The informational shell from the big bang itself should be long gone by most mundane understanding of physics.

The drawing sure look like those two aluminum bars at 90 degrees are a lot longer then 10M (meters) .Maybe 10km which makes more sense since they weigh 7600 kg each. Heaven forbid the M stands for miles ! That's how we crashed into Mars.

The drawing sure look like those two aluminum bars at 90 degrees are a lot longer then 10M (meters) .Maybe 10km which makes more sense since they weigh 7600 kg each. Heaven forbid the M stands for miles ! That's how we crashed into Mars.

What drawing are you talking about? If you mean the photograph at the beginning of the article, that is of the LIGO detector, and is clearly labelled as such. It is not supposed to indicate what a torsion bar detector would look like.

Also, 10km for 7600kg would make more sense?? Just how light do you think aluminium is? Aluminium has a specific gravity of about 2.7 (which means that 1 cubic metre of aluminium weighs about 2700kg), so a 10m bar would only need a cross section of roughly 0.28m^2 to weigh 7600kg*. Making it 10km long would result not in a bar, but a wire of only a few millimetres in diameter.

I only take issue with the interference. Since both bars cannot be mounted with the same center of mass, (some 5m apart, minimum) terrestrial waves will interfere. However, these should differ by order of magnitude and additional measurements should be able to measure the interference and back it out.

They are stacked on top of each other, so looking down on them, they appear as a cross. This makes the center-center distance just over 0.5m. Of course they are most sensitive to in-plane gravity waves.

This ain't my area of expertise, so do excuse if this sounds utterly idiotic, but: How do we know that gravity waves would propagate at the speed of light? Why exactly should we assume that it does? We're not talking about a a stream of particles moving *through* a certain medium (such as photons moving through the universe), we're talking about the medium *itself* fluctuating. What's to say that gravity waves (if they exist) don't propagate at speeds perhaps arbitrarily greater than the speed of light? Also, considering that "speed" is partially dependent on time, which is generally considered to be an integral part of the *spacetime* medium, would "speed" even be a meaningful way to represent the propagation of said waves?

I know its not very cool to question Einstein. However, I am looking at the following quote from the above article:

"First questions first: how do we know that gravity waves exist? The general theory of relativity, which is, at heart, a theory of gravity, predicts them. In general relativity, changes in mass at a location cause space and time to stretch and compress."

"Causes space and time to stretch and compress" ? This is meaningless nonsense, surely? Why is this "explanation" any less unsatisfactory than the idea of a gravitational field? Both theories seem to posit the existence of something in what looks to me like nothing but empty space!

Light travels at the speed of light because of EM interactions and this was worked out by Maxwell.A gravitational wave is probably a compression wave and a graviton would be completely different from a photon.We will have to hope it travels at the speed of light.Because if it doesn't; I worked this out last week for a speed of gravity quite different for C; if the wave is at terrahertz frequency then the quarter wavelength antenna involved runs on the order of 450 light years long.For a very very high value of Gv.Making the wave damn hard to detect.So, hope for light speed.

I know its not very cool to question Einstein. However, I am looking at the following quote from the above article:

"First questions first: how do we know that gravity waves exist? The general theory of relativity, which is, at heart, a theory of gravity, predicts them. In general relativity, changes in mass at a location cause space and time to stretch and compress."

"Causes space and time to stretch and compress" ? This is meaningless nonsense, surely? Why is this "explanation" any less unsatisfactory than the idea of a gravitational field? Both theories seem to posit the existence of something in what looks to me like nothing but empty space!

OK, snarky reply removed in favor of a simple answer.

This explanation is more satisfactory because it is part of a model which is quite probably the single most frequently, rigorously, and precisely verified scientific construct in history. General Relativity has been tested to extraordinary degrees of accuracy, over and over, in innumerable contexts, and the results have always been consistent with its predictions.

The notion of space and time stretching and compressing is as far from "meaningless nonsense" as it is possible to get. It has more explanatory and predictive power than any other going theory, and is supported by an immense body of experimental data.

ooh. Cheap == can make a whole bunch of them == . . . what if synthetic aperture works?

OK - we still couldn't make a 450 million light-year long antenna, but if they DO travel at C, then using synthetic aperture techniques, maybe we could get some better sensitivity and resolution out of an array of these?

I know its not very cool to question Einstein. However, I am looking at the following quote from the above article:

"First questions first: how do we know that gravity waves exist? The general theory of relativity, which is, at heart, a theory of gravity, predicts them. In general relativity, changes in mass at a location cause space and time to stretch and compress."

"Causes space and time to stretch and compress" ? This is meaningless nonsense, surely? Why is this "explanation" any less unsatisfactory than the idea of a gravitational field? Both theories seem to posit the existence of something in what looks to me like nothing but empty space!

Simplest thing to do conceptually is look at the absorption lines from the Sun and look at the absorption lines of the same elements in the lab: there is a small but present gravitational redshift happening. That shows time being stretched out by gravity.

icecycle wrote:

Light travels at the speed of light because of EM interactions and this was worked out by Maxwell.A gravitational wave is probably a compression wave and a graviton would be completely different from a photon.We will have to hope it travels at the speed of light.Because if it doesn't; I worked this out last week for a speed of gravity quite different for C; if the wave is at terrahertz frequency then the quarter wavelength antenna involved runs on the order of 450 light years long.For a very very high value of Gv.Making the wave damn hard to detect.So, hope for light speed.

How do you come by a speed of 4x450 light years x 1THz ~ 5.68e22 c?

There's an awful lot of reference to light beams and their speed when it comes to relativity, but what I find helpful conceptually is to substitute "the magnitude of the invariant relative velocity" for it. Photons happen to travel at this speed because they are massless; likewise gravity's force-carrier does the same. It doesn't seem very likely, but it's not utterly impossible that we should one day find that the photon particle has a tiny, tiny, tiny mass. If that should happen to be the case then it would bring into sharp contrast the distinction between c, the magnitude of the invariant relative velocity as used in relativity and the speed of the EM force carrier.

There's not that much hoping required that the speed of gravity = c owing to the match of the binary pulsar timing that lethe mentioned to the GR prediction that the power radiated is proportional to the inverse of the fifth power of the speed of gravity. The observations (see eqn 63 and compare to the GR-derived equations with a bit of treatment) match the speeds of light and gravity to within about 0.1%.

If you want the speed of gravity to be higher than c then you'll have to live with the conclusion that it would then be possible to build a gravitational device to signal one's own past plus you would need an alternate theory to explain the pulsar timing data at least as well as GR; if you want the speed of gravity to be lower than c then you'll have to live with the fact that it must be almost identical lest ultra-high-energy cosmic rays be slowed below their observed energies by gravitational Čerenkov radiation, see here.

The evidence is consistent with the speed of gravity being exactly the same as that of light, i.e. the magnitude of the invariant relative velocity.

If you want the speed of gravity to be higher than c then you'll have to live with the conclusion that it would then be possible to build a gravitational device to signal one's own past plus you would need an alternate theory to explain the pulsar timing data at least as well as GR; if you want the speed of gravity to be lower than c then you'll have to live with the fact that it must be almost identical lest ultra-high-energy cosmic rays be slowed below their observed energies by gravitational Čerenkov radiation, see here.

The evidence is consistent with the speed of gravity being exactly the same as that of light, i.e. the magnitude of the invariant relative velocity.

I can't reasonably argue with most of your last post, however...

I'm not fully convinced that the ability to signal one's past would have to be accepted to allow for FTL propagation of gravity. I don't yet have a fully fleshed out answer, but something about this assumption just doesn't *sit right* with me. Consider the following:

The argument that nothing can move faster than light is ultimately based on causality; time comes to a halt for an object moving at c, and thus would (presumably) reverse for an object exceeding this speed, with the dizzying potential to violate the all-important causality. If we break this down to natural units (in this case, Planck units) we understand that a photon (or other massless particle, perhaps a graviton) travels one Planck Distance for every Planck Time (assuming a gravitationally neutral vacuum).

With this in mind, imagine that a particle *does* possess the necessary qualities to travel faster than c, and thus (again, presumably) backwards in time. It's my belief (that I've yet to prove mathematically) that such a particle would only be detectable in any given moment in the past for precisely *one* Plank Time, regardless of its superluminal speed (i.e. a particle moving at 2c would be visible for the same duration of time that a particle moving 100c would be... they'd simply be visible at different times in the past if emitted at the same point in the future.).

Assuming this is accurate, then particles from the future wouldn't exist nearly long enough for any meaningful signal to be discerned. Such particles would appear to blink into -- and then out of -- existence in a completely random fashion. In a similar sense as with quantum entanglement, the fact that no actual *information* can be exchanged protects any causal violation that one might otherwise expect.

Superluminal particles, waves, or other such phenomena would essentially manifest as virtual particles -- bound by the uncertainty principle. Furthermore, if we consider the idea that gravity and/or other particles/waves could exceed c (tachyons, I'm looking at you), then at any point in time, we'd observe a universe filled with particles and energies that were originally emitted at any range of times in the future. Such a large volume of continuously coming-and-going, interacting-then-vanishing particles *MIGHT* be observed as dark matter and/or dark energy. At any given moment, the universe is *filled* with these randomly occurring particles, and exactly one Planck Time later, the universe would be filled with an *entirely* new (and more or less energy/mass equal) sea of these particles.

Like I said, I haven't done enough work on this hypothesis to say for certain that I'm not just talking out of my ass. Also, I really can't covey what I'm trying to say as well as I'd like, but what I have going on in my head strikes me as both incredibly simple, and highly compelling. I will continue to work on this concept...